The Lochaber marathon ends up back at Fort William, so from there it an easy jog back down the West Highland Way should see you back in Milngavie by Monday lunchtime. Alternatively, you could run up the Great Glen Way to Inverness, along the coast a bit then take the Speyside Way south, through the Lairig Ghru to Braemar, over the Mounth (lots of routes to choose from), round the Fife Coastal Path then along the Forth-Clyde Canal back to the centre of Glasgow - then it is a short train ride to Milngavie.